Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on dielectrics, conductors and resistors for this use. These are conventionally thick film materials which are applied to substrates by screen printing in the desired pattern. The material is then heated, i.e., "fired," to burn out all the organic materials and sinter the inorganic materials. The problem with this approach has been the limited resolution capability of thick film screen printing. Typically, it is difficult to obtain better than 8 mil (200 micrometer) line and space resolution or better than 10-15 mil (250-375 micrometer) diameter via holes.
An alternative approach is to use photosensitive materials. The polymeric material is applied to the substrate in a photosensitive medium comprising a monomer, a binder and a photoinitiator. The layer is then exposed to actinic radiation in an imagewise pattern, i.e., through a phototool, and developed to remove the unexposed areas. For electronic applications, the photosensitive layer also contains the functional material, i.e., dielectric, conductor or resistor. After development of this layer, it is fired as described above. However, there are problems getting adequate contact between the photosensitive layer and the phototool without having the phototool scratch the surface of the photosensitive layer. The photosensitive materials generally require development by organic solvents, which may result in handling and waste disposal difficulties. In addition, the photoinitiators which are used in photosensitive compositions are frequently sensitive to the room light. Thus, these materials must be handled in a yellow light environment to prevent photohardening initiated by room light.
This difficulty can be overcome using a diffusion patterning process as disclosed by Felten in U.S. Pat. No. 5,032,216. In this method, a first layer of nonphotosensitive polymeric material is applied to a substrate in an unpatterned manner, followed by a second nonphotosensitive layer in a pattern. The second, patterning layer diffuses down to the first layer effecting a change in the dispersibility of that layer. The first layer is then removed in those areas which have greater dispersibility. Again, for electronic applications, the functional material is contained in the first layer, which is fired after development.
While diffusion patterning is versatile, fast and economical, it does have some limitations. In particular, the diffusing material has a tendency to spread in the X-Y direction as it diffuses in the Z direction (vertically). This leads to a loss in resolution and makes it difficult to make very small via holes or other openings.
There exists a need, therefore, for an improved process for making very fine patterns in organic polymer films, for use in electronic applications. In addition, it would be desirable to have materials which can be developed in aqueous solutions or in water alone.